1. Field of the Invention
The present invention relates to a thermal transfer recording method in which the heat-generating elements of a thermal head are selectively heated to transfer the ink on an ink film to a sheet of print paper for recording and a thermal transfer printer based on the method and, and more particularly, to the improvements in a thermal transfer recording method capable of recording on dedicated print paper a color image of high picture quality comparable to a silver salt photograph and a thermal transfer printer based on this method.
2. Description of the Related Art
In prior-art thermal transfer printers in general, a sheet of print paper is held in front of a platen, a thermal head formed with a plurality of heat-generating elements is mounted on a carriage, an ink film with ink applied to the base, for example, an ink ribbon, and the print paper are held between the thermal head and the platen, the ink ribbon is fed while the thermal head is being reciprocated on the carriage along the platen, the heat-generating elements on the thermal head are selectively supplied with an electric current according to recording information, and the ink on the ink ribbon is partially transferred by the heat generated on the selected heat-generating element onto the paper for recording desired images such as characters. Such thermal transfer printers are often used as output devices of computers and dedicated wordprocessors for their high recording picture quality, low noise, low cost, and ease of maintenance.
One of well-known conventional thermal transfer printers uses an ink ribbon with a heat-melt ink applied to the base such as a plastic film (this ribbon is referred to as a heat-melt ink ribbon) for recording, while another uses an ink ribbon with a heat-sublimation ink applied to the base (this ribbon is referred to as a heat-sublimation ink ribbon) for recording.
The thermal transfer printer that uses the heat-melt ink ribbon (hereinafter referred to as a heat-melt type printer) is mainly used to print on a wide variety of papers such as plain paper, thick paper, and postcard, and excellent in use. For gray-scale image printing with the heat-melt ink ribbon, dither process is used because gray-scale control on a dot basis is disabled with this type of printer.
On the other hand, the thermal transfer printer that uses the heat-sublimation ink ribbon (hereinafter referred to as a heat-sublimation type printer) can adjust the sublimation quantity of the heat-sublimation ink to control the ink quantity to be transferred to the paper by regulating the energy to be applied to the thermal head for heating the same. Therefore, the printer of this type can provide full-color recording image printing comparable in quality to silver salt photography if a dedicated surface-processed print paper is used. For this merit, the heat-sublimation type printer has recently come to be widely used as a video printer of high picture quality for example.
A thermal transfer printer that can use the ink ribbons of both types has been developed. A full-color recording printer implements full-color printing by combinations of three colors of yellow, magenta and cyan, or combinations of four colors of yellow, magenta, cyan and black. Generally, for the heat-sublimation type printer to provide a high-density recording image, a higher energy is required for driving the heat-generating elements of the thermal head than on the heat-melt type printer.
In the above-mentioned heat-sublimation type printer, printing an image composed of two colors or less by applying a large amount of energy to the heat-generating elements of the thermal head does not adversely affect the print paper, providing good printing. However, printing a high-density gray recording image with the above-mentioned three or four colors by applying the same amount of energy thermally deforms the print paper to cause a so-called matting phenomenon such as wrinkle or wave, failing to provide good-quality printing. The print paper is thermally deformed by a total accumulated energy given from the heat-generating elements every time the ink of a single color is applied to the paper, exceeding a threshold of paper thermal deformation.
In the above-mentioned conventional thermal transfer printers, printing with a heat-sublimation ink ribbon may cause discoloration of the print due to ultraviolet ray and so on if the print is left in the open air for long because the dye used in the ink is low in weatherability, and peeling-off of the transferred ink when it is scratched, making it difficult to maintain print quality over a long period of time.